The present invention relates to treatment planning for radiation therapy of tumors and the like and in particular for treatment planning system providing an improved assessment of the proton stopping power of tissue being treated.
External-beam radiation therapy may treat a tumor within a patient by directing high-energy radiation in one or more beams toward the tumor. Selective disruption of tumor tissue is obtained both by concentrating radiation at the site of the tumor and by taking advantage of a greater susceptibility of tumor tissue to radiation, for example, resulting from its higher cell division rate.
Photon external-beam radiation systems, for example, as manufactured by Accuray, Inc., of Sunnyvale Calif., treat a tumor with multiple x-ray fan beams directed at the patient over an angular range of 360°. Each of the beams is comprised of individually modulated “beamlets” whose intensities can be controlled so that the combined effect of the beamlets, over the range of angles, allows complex treatment areas and dose patterns to be defined.
X-ray photons deposit energy in tissue along the entire path between the entrance and the exit point in the patient, necessarily irradiating healthy tissue along the path to the tumor. This drawback has suggested the use of heavy charged particles, such as protons alpha particles or carbon ions, as a substitute for x-ray photon radiation. Unlike x-rays, protons or heavier ions may be controlled to stop within the tissue, reducing or eliminating exit dose through healthy tissue on the far side of the tumor. Further, the dose deposited by a proton beam is not uniform along the entrance path of the beam, but rises substantially to a “Bragg peak” near a point where the proton beam stops within the tissue. By positioning the Bragg peak inside the tumor, the entrance dose through healthy tissue may be substantially reduced with respect to the dose deposited in the tumor.
The accuracy possible with proton beam therapy is best exploited if a treatment plan is prepared tightly defining the dose to the patient with respect to a precise characterization of the patient tissue such as may be obtained from an image of the patient tissue, for example a set of CT slices covering the tumor area. A treatment plan tied to such images may permit a physician to accurately demarcate within the images a region to be treated and a dose within that region. The identified region may then be used to calculate the orientation, energy and intensity of multiple proton beams that will produce the demarcated dose and treatment area.
Converting a description of the treatment area and dose to orientation, energy, and intensity of multiple proton beams requires an understanding of the proton stopping power of the tissue along the path of the protons. This knowledge allows adjustment of the energy of the proton beam to position the Bragg peak precisely within the tumor site.
A conventional x-ray CT image can provide information about the attenuation of x-ray photons but is a relatively poor proxy for proton stopping power leading to errors of 3 to 5 percent in practice employing the best known conversion methods. It is known that generating tomographic images using the proton beam itself instead of x-ray photons is a more accurate way of obtaining proton stopping power information. Protons entering and exiting the patient are detected by position and direction detectors (for example silicon strip detectors), and then collected by an energy detector (for example a scintillation detector) for residual energy detection. Such detected proton data form a proton sinogram that describes the summation of proton stopping powers for different proton beamlets across the patient at a range of beam angles. The proton sinogram is further used to reconstruct a tomographic image that contains the proton stopping power values of each spatial location of the body tissues within the patient. This tomographic image is used for proton therapy treatment planning.
A drawback to this approach arises from the limited energy of therapeutic proton beams of approximately 200-250 MeV. This proton energy is insufficient for the protons to traverse more than approximately 25 to 38 centimeters of tissue before entering the Bragg peak region at which point the either deposit significant energy or stop in the tissue, and therefore, for many patients, the image sinogram provided by the proton beam will be incomplete, leading to severe image artifacts that obscure the quantitative data that must be extracted for treatment planning.
Methods of reconstructing tomographic images when the underlying sinogram is incomplete are known in the x-ray CT art, however, the large and continuous regions of missing data likely in a proton sinogram, and the need for quantitative accuracy in the proton sinogram, make these x-ray CT techniques generally inapplicable.